One reason: Plowing large areas of land
A few more reasons: poor farming practices, farmers would loosen up the soil, and the land was deforested, making it loose the roots that held the soil in place.
The decay equation is of the form

where
m(t) = the mass remaining after t minutes
m₀ = the initial mass
k = constant
Because the half-life is 3 minutes, therefore

Therefore
-3k = ln(0.5)
k = ln(0.5)/(-3) = 0.231
Because m₀ = 16 mg, the time, t, for the mass to decay to 1.0 mg is given by

Answer: 22 min (nearest integer)